Optically readable disc with a centering member fixed to a transparent substrate

ABSTRACT

A transparent substrate has an information layer on an entirely flat side, and a centering member on the other side adapted to cooperate with a centering device of an apparatus for inscribing and/or reading the disc. The centering member may be glued in place and have an aperture for engagement by an apparatus spindle.

BACKGROUND OF THE INVENTION

The invention relates to an optically readable disc comprising a roundtransparent substrate to be driven about a central axis of rotation, andan information layer.

Such discs are known in various types and versions. For example, BritishPatent Specification No. 1,446,009 (herewith incorporated by reference)describes optically readable video discs comprising a transparentsubstrate which is provided with a structure on one side which can beread through the substrate by means of a laser beam, and which is coatedwith a reflecting vacuum-deposited metal layer coated with a protectivelayer of lacquer. Frequently, two such discs are glued to each otherback-to-back, yielding a double disc which is optically readable on bothsides. For centering and positioning the video disc on a video-discplayer, the disc is provided with a central aperture which cooperateswith a centering member arranged on a drive spindle in the video-discplayer. Optically readable audio discs generally have small dimensionsand a differently coded information structure, but they are otherwise inprinciple identical to these known video discs.

Another type of optically readable disc is known from U.S. Pat. No.4,074,282 (herewith incorporated by reference). These known discscomprise one or more transparent substrates--sometimes in combinationwith a non-transparent disc--which are glued to each other by means ofcoaxial annular spacers. By means of the annular spacers at least onegas-tight space is formed between the various parts of the disc. Thistype of optically readable disc is intended as a storage disc. By meansof a laser beam a recording layer adjoining a sealed space can beprovided with information through a transparent substrate. The recordinglayer comprises, for example, a thin layer of a tellurium alloy formedon the substrate by vacuum deposition.

Still other known optically readable discs are used as an intermediateproduct in the manufacture of optically readable video discs and audiodiscs. These otpically readable discs, generally referred to as"masters", comprise a glass substrate on which a photosensitive layer isdeposited on one side. In a highly dust-free and very well ventilatedroom the video information and/or audio information is inscribed in therecording layer by means of a laser beam using special precisionequipment. After the information has been recorded the photosensitivelayer is developed. The photosensitive layer then contains a pattern ofpits which can be duplicated by means of duplicating techniques of thetype as described in for example U.S. Pat. No. 4,312,823 (herewithincorporated by reference). The masters are provided with a centralaperture for accurate centering on the drive spindle of the machine usedfor recording the information.

The optical storage discs as generally used until now comprise two glasssubstrates which are glued to each other with interposed concentricannular spacers. However, when depositing the tellurium layer on theseglass substrates and when depositing the photosensitive layer on theglass substrates of masters some problems occur. These problems aremainly associated with the method of depositing these layers on thesubstrates. By way of example a brief description will be given of themethod used for depositing the photosensitive layer on the masters andthe further processing of the exposed disc.

A ground glass substrate is employed whose surface, which is to beprovided with the photosensitive layer, is first thoroughly cleaned andwashed with demineralized water. These operations and all furtheroperations to which the masters are subjected must be carried out in aconditioned highly dust-free room. For cleaning and washing thesubstrate is placed on a rotating spindle which projects through thecentral hole. The disc is dried by rotating it with a higher speed, sothat the water particles are spun off the surface. The next steps in themethod are also effected with a rotating disc, repeated drying beingeffected by rotation with a higher speed. After having been washed anddried the substrate is degreased by means of a suitable liquid, afterwhich an adhesive liquid is applied to the disc. Then degreasing isrepeated and finally the photoresist layer is applied. The photoresistlayer is distributed evenly over the surface by fast rotation of thedisc. After the exposure of the photo-resist layer on the describedabove, special machine developing takes place, again with the discrotating, after which in a final operation the surface is rinsed againand subsequently a silver layer is applied by vacuum deposition. It hasbeen found that it is difficult to prevent residual liquids used inprevious steps of the method from settling in the central aperture ofthe disc. This may cause contamination of the product and consequentrejection of the product. Without describing the entire method ofmanufacturing optical storage discs it will be evident from theforegoing that, since the substrates of such discs also come severaltimes in contact with liquids, similar problems will occur duringmanufacture.

Even with video discs and audio discs the central disc hole may giverise to problems. As already stated, the discs are provided with anextremely thin metal layer, in general vacuum-deposited silver, which iscovered with a lacquer coating of 5 to 10 microns thickness. The silverlayer must be coated very thoroughly because in the long run it may beaffected by sulphur from the atmosphere, thereby rendering the discunserviceable. The lacquer coating extends from the circumference of thedisc to the central hole. At these locations there is an increased riskof the silver layer being attacked. Near the center hole this risk evenincreases because the disc is clamped in position near the central hole,so that at this location forces are exerted on the disc which may giverise to shear stresses between the lacquer coating and the silver layer.

SUMMARY OF THE INVENTION

It is the object of the invention to provide optically readable discs ofthe type mentioned in the opening paragraph which do not present theaforementioned problems.

According to the invention the substrate comprises a disc withoutaperture. The disc is flat at least on the side where the informationlayer is located, the substrate carries an axially projecting centralcentering member on the other side for cooperation with centering meansof an apparatus for incscribing and/or reading optically readable discs.

All the aforementioned problems which evolve from the presence of thecenter hole are solved by the invention. However, depending on theembodiment, the invention also has some further advantages. Wheninjection molding-plastic audio and video discs a moving part in theinjection-molding machine for the formation of the center hole may bedispensed with. Alternatively the plastic disc may be provided with anintegrally molded plastic central centering member. This is alsoexpected to have the advantage of reducing stresses in and consequentdeformations of the disc.

An advantageous embodiment of the invention is characterized in that thecentering member at least partly comprises a ferromagnetic material. Inthis respect it is advantageous, in particular when the substrate ismade of glass but also in other cases, if the entire centering member ismade of a ferromagnetic material and is connected to the substrate by alayer of glue. The advantage of these embodiments is that the disc canbe clamped magnetically on a drive spindle. It is then no longernecessary to urge the disc in position from the other side or to clampit in position by other mechanical means. Moreover, in the manufactureof the masters this yields the additional advantage that theaforementioned method can be automated, the rotating spindles used inthe various stage of manufacture being provided with an electricallycontrolled electromagnetic clutch which releases or attracts thecentering member, as desired. As a result of this the substrate can betransferred automatically from one spindle to another by mechanicalmeans, so that it need not be handled by an operator. This leads notonly to an increase of the productivity but also to a cleaner process,resulting in a further reduction of the percentage of rejects.

If the centering member comprises a separate component which is affixedto the disc, optical means may be used for the accurate alignment of thecentering member relative to the tracks recorded on the disc or relativeto the outer circumference of the disc.

An advantageous embodiment, which is particularly suitable for consumerproducts such as small storage discs, is characterized in that thecentering member comprises a part which is made of a sheet material andwhich has a flat round bottom which adjoins a flat flange-shaped rim,and the flange-shaped rim is connected to the substrate by the layer ofglue. Such a centering member can be manufactured simply and cheaply andis found to perform satisfactorily in practice. The apparatus on whichthe disc is used may be provided with a drive spindle havingpermanent-magnetic means which cooperate with the sheet-steel centeringmember. For accurately and simply centering the disc on a drive spindleit is advantageous to use another embodiment, which is characterized inthat the bottom of the centering member is formed with a centralcentering aperture for the passage, with a slight clearance, of acentering means of an apparatus for inscribing and/or reading opticallyreadable discs. In the sheet steel component a central aperture can beformed with high precision and with a smaller tolerance on its diameter.Moreover, a central aperture is very suitable for glueing the centeringmember to the substrate at the correct location with the aid of opticalmeans.

The invention will now be described in more detail, by way of examplewith reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a perspective view of an optically readable storagedisc in accordance with the invention, a part of the storage dis andsome parts of an apparatus for driving the storage disc being cut away.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The storage disc comprises a transparent glass or plastic substrate 1. Areflecting recording layer 2, which can be modified locally by means ofa radiation beam, is deposited on the substrate. In the presentembodiment this recording layer is present only at the substrate areawhich is actually intended for recording. However, it is alternativelypossible to deposit the recording layer over a larger part of thesubstrate, for example over the entire substrate area. Information isrecorded and read through the transparent substrate by means of a laserbeam, not shown. A cover disc 3 is fixed to the substrate in asubstantially gas-tight manner at some distance from the recording layer2, so that a sealed space 4 is enclosed between the substrate and thecover disc. The cover disc is substantially thinner than the substrateand has a substantially smaller resistance to such deformations as maybe caused by differences between the gas pressure in the sealed space 4and the barometric pressure.

The cover disc is made of thin metal sheet, for example aluminum sheet,which is formed into a shallow tray comprising first parts 3A in theform of an annular flange disposed in a first plane for fixing to thesubstrate. Second parts 3B constituting the bottom of the shallow trayand covering the sealed space 4 are disposed in a second plane parallelto the plane in which the flange 3A is disposed. Adjacent the flange 3Athird parts are present in the form of a turned-over rim 3C comprisingthe edge of the substrate. These third parts have been spun or rolledaround the edge of the substrate. The substrate 1 comprises a perfectlyflat round disc and is not formed with a central aperture for centeringand fixing the disc on a drive means.

The cover disc is only connected to the substrate at the location of thecircumference. The cover disc comprises fourth parts 3D in the form of around embossment whose bottom is disposed in a plan between theaforementioned two planes and above a part of the substrate which is notused for recording. This part is therefore situated at some distancefrom the substrate but is situated closer to the substrate than the part3B. In case of depression of the cover disc this part, inter aliabecause it is the most central part, will first come in contact with thesubstrate in an area where this has no undesired consequences. In thisrespect it is important, especially if the metal sheet used is thin,that the cover disc is provided with radial reinforcements 3E obtainedby local deformation. These ensure that the part 3B has the requiredrigidity.

The storage disc can be rotated about a central axis of rotation 5. Onthe side facing away from the recording layer 2 the substrate carries acentral centering member 6 which projects in the axial direction. Thismember is adapted to cooperate with centering means in the form of aspindle 7 of an apparatus, not shown, for inscribing and/or readingoptically readable storage discs. A suitable apparatus, apart from thecentering means which have a slightly different shape and which areintended for discs having a central aperture, is described in U.S. Pat.No. 4,545,046 (herewith incorporated by reference). The entire centeringmember 6 is made of sheet steel and is connected to the substrate by alayer of glue 8. It has a flat round bottom 6A and a flat flange-likerim 6B which is connected to the substrate by the layer of glue 8. Thebottom 6A of the centering member 6 is formed with a central centeringaperture 6C through which the spindle 7 extends with a slight clearance.This spindle may be the motor spindle of a drive motor, allowing a verysimple, yet sufficiently accurate, centering of the storage disc on thedrive means.

The spindle 7 carries a turntable 9 which supports the substrate 1. Theturntable carries an annular permanent magnet 10 which draws the sheetsteel centering means 6 axially onto the turntable 9. Moreover, theturntable is provided with a ring 11 of a suitable elastic materialwhich is subject to a slight elastic deformation as a result of theforce of attraction of the permanent magnet. In the present embodimentthe ring 11 cooperates directly with the substrate 1. Alternatively, theflange 6B of the centering means 6 may be given a larger diameter, sothat the ring 6 does not cooperate directly with the substrate 1 butwith the flange 6B. The free end of the motor spindle 7 bears againstthe underside of the substrate 1 and thereby constitutes a stop whichdefines the axial position of the information disc on the turntable 9.

The invention is not limited to the embodiment shown but many differentembodiments are possible within the scope of the invention as defined inthe claims. For example, a centering member may be used which comprisesa plurality of parts, which may be made of different materials. A partof the centering member may then be integral with the substrate.

What is claimed is:
 1. An optically readable disc adapted to be drivenabout a central axis of rotation, comprising a round transparentsubstrate and an information layer disposed to one side of thesubstrate,characterized in that the substrate comprises a substantiallyrigid disc without aperture, said disc being flat at least on the sidewhere the information layer is disposed, and an axially projectingcentral centering member of ferromagnetic material disposed on the otherside of the substrate and joined thereto by a layer of glue, saidcentering member cooperating with centering means of an apparatus forinscribing and/or reading optically readable discs.
 2. An opticallyreadable disc adapted to be driven about a central axis of rotation,comprising a round transparent substrate and an information layerdeposited on one side of the substrate,characterized in that thesubstrate comprises a substantially rigid disc without aperture, saiddisc being flat at least on the side where the information layer isdeposited, and an axially projecting central centering member madeentirely of a ferromagnetic material, disposed on the other side of thesubstrate, for cooperating with centering means of an apparatus forinscribing and/or reading optically readable discs, and comprising apart made of a sheet material, having a flat round bottom and a flatflange-shaped rim adjoining said bottom, said rim being connected to thesubstrate by a layer of glue.
 3. An optically readable disc as claimedin claim 2, characterized in that the bottom (6A) of the centeringmember (6) is formed with a central centering aperture (6C) for thepassage, with slight clearance, of a centering means (7) of an apparatusfor inscribing and/or reading optically readable discs.